This invention relates to transistors and, more particularly, to heterojunction bipolar transistors (HBTs).
In a typical HBT the emitter is made of a wider bandgap semiconductor than the base in order to increase the emitter injection efficiency and to prevent minority carrier injection from the base into the emitter. Although HBTs can be fabricated from a variety of materials (e.g., Group III-V compounds), most attention is focused on attaining high speed operation of devices in the AlGaAs/GaAs materials system. The switching speed of an HBT, like many other semiconductor devices, is limited by parasitic resistance (R) and capacitance (C). Thus, the rapid progress recently made in increasing the operating speed of AlGaAs/GaAs HBTs can be attributed mainly to a reduction in parasitic RC time constants; that is, by reducing lateral dimensions using various self-aligned processes, by reducing the collector capacitance with buried ion implantation, and by lowering the emitter contact resistance using a non-alloyed emitter contact on an epitaxial n+-InGaAs contact-facilitating layer. Even with the success of these techniques, however, there still exists a fundamental limit on the speed of the HBT which is governed by the emitter-collector transit time delay associated with the electron velocity through the emitter, base, and collector regions.
One of the major delays is caused by the base transit time .gamma..sub.B which is determined by carrier diffusion through the neutral base region and can be expressed as EQU .gamma..sub.B =W.sub.B.sup.2 /2D.sub.e ( 1)
where W.sub.B is the base width and D.sub.e is the electron minority carrier diffusion coefficient. One way to decrease the base transit time is to introduce a quasi-electric field by grading the alloy composition (and hence bandgap) in the base to increase the electron velocity. It has been shown experimentally that the base transit time can be significantly reduced by alloy grading in the base, in which case the base transit time is given by EQU .gamma..sub.B =W.sub.B /V.sub.e =W.sub.B /.mu..sub.e F (2)
where v.sub.e is the electron velocity, .mu..sub.e is the electron mobility, and F is the quasi-electric field in the base. It should be noted for thin based with W.sub.B &lt;0.1 .mu.m, equations (1) and (29 do not strictly apply due to boundary value problems and non-equilibrium transport effects. Nevertheless, it is clear that in the limiting case of vanishing base-width, the base transit time goes to zero: ##EQU1##
A few bipolar-type transistors have been proposed which contain a voltage-induced two-dimensional hole base such as the biplar inversion channel field effect transistor (BICFET) described by G. W. Taylor et al, IEEE Trans. Electron Dev., Vol. ED-32, p. 2345 (1985) and the inversion base bipolar transistor (IBT) described by K. Matsumoto et al, IEEE Electron Dev. Lett., Vol. EDL-7, p. 627 (1986). Both devices have effective base widths below 100 .ANG.. However, these transistors do not have a neutral base region and hence have a high base resistance and require self-alignment of the base and emitter contacts.